1. Field of the Invention
The invention relates to a method for manufacturing an epitaxial wafer, and in particular, relates to a Group III metal nitride epitaxial wafer serving as a laser diode substrate.
2. Description of the Related Art
In recent years, the research into Group III and V nitrides has drawn much attention. Especially, after Nichia Chemical Corporation in Japan claimed in 1993 that high-brightness blue light emitting diodes would be mass produced, most related studies have focused on the research of light emitting diodes. While there have been some breakthroughs regarding laser diodes, no substrates have been found having a lattice constant that matches that of the active layer, thereby reducing a defect density. Therefore, finding a suitable substrate represents a bottle neck for the growth of laser diodes.
Generally, the defect density of a gallium nitride layer formed on a sapphire substrate is about 10.sup.7 -10.sup.10 cm.sup.-2. These materials have a lattice mismatch that is as great as 14%. This prevents gallium nitride from being used as an active layer of a laser diode. Although gallium nitride laser diodes have been developed, their lifetimes are too short.
Consequently, there is still room for improvement. Thus, how to find lattice-matching substrates is an important subject of research. In addition to single crystal sapphire (Al.sub.2 O.sub.3) substrates, silicon carbide substrates are also considered as an alternative choice. However, silicon carbide is expensive, and is not suitable for mass production. Furthermore, since the lattice mismatch between silicon carbide and gallium nitride is 3.5%, complete lattice matching can not be attained. Therefore, a need remains for substrates that have a better lattice matching with Group III metal nitride and are easily obtained.
One possible solution could be to directly manufacture gallium nitride substrates instead of finding hetero-epitaxial substrates. Unfortunately, the saturated vapor pressure of nitrogen in gallium nitride is too high. It is not easy to manufacture single crystal gallium nitride by pulling a single crystal. Moreover, the temperature needed to grow single crystal gallium nitride is over 1800.degree. C. The pressure required during growing is 20 thousand atmospheres in order to keep nitrogen inside the single crystal gallium nitride. Therefore, it is not an appropriate method for growing the crystal.
In 1990, gallium nitride single crystal was grown on sapphire substrates by using a traditional HVPE method by Nagoya University in Japan. However, the surface of the formed gallium nitride was not really flat, and the characteristics of the gallium nitride were not improved. For improving the surface of the single crystal gallium nitride, a method similar to that of growing a gallium nitride epitaxial layer on a sapphire substrate by an MOCVD technique is adopted. In this method, zinc oxide is first sputtered on a sapphire substrate, serving as a buffer layer. After that, single crystal gallium nitride is formed by an HVPE method. However, the resulting improvements still fall short of expectations.
In growing gallium nitride (GaN) by an HVPE method, hydrogen chloride (HCl) first passes through the surface of gallium having a high temperature of over 800.degree. C., thereby forming a vapor of gallium chloride (GaCl). Then, the gallium chloride reacts with ammonia (NH.sub.3) at a temperature of 1000-1200.degree. C. to form gallium nitride which is then deposited on the substrate to form a gallium nitride epitaxial layer. The reaction equation is given as follows: EQU 2HCl+2Ga.fwdarw.2GaCl+H.sub.2 EQU 2GaCl+2NH.sub.3 +H.sub.2 .fwdarw.2Ga+2NH.sub.4 Cl
The above-mentioned method for manufacturing an epitaxial wafer according to the prior art has the following disadvantages:
1. In addition to the white ammonium chloride (NH.sub.4 Cl) powder created from the reaction of gallium chloride and ammonia, excessive or non-reaction hydrogen chloride and ammonia can create a large amount of additional ammonium chloride (NH.sub.4 Cl) powder. This can affect the surface flatness of the gallium nitride epitaxial layer. Further, the lifetime of the air pumping system can be shortened due to the large amount of powder. PA1 2. Hydrogen chloride is a toxic gas with high corrosion. Human bodies can be harmed upon exposure to only 5 ppm of hydrogen chloride. Also, there can be a strong corrosive action to the system.
As described above, prior art methods cannot provide a perfect Group III metal nitride epitaxial wafer. So, a new method to overcome the above-mentioned problems and efficiently provide a perfect Group III metal nitride chip is expected by industries.